1. Field of the Invention
The present invention relates to a process for producing a polyurethane emulsion having excellent storage stability by continuously emulsifying a urethane prepolymer containing substantially no organic solvent with water and reacting the urethane prepolymer with a chain extender.
This application is filed under 35 USC 371 based on the National Stage of International Application No. PCT/JP03/03627, filed on Mar. 25, 2003, the content of which is incorporated herein by reference. Priority is claimed on Japanese Patent Application No. 2002-91266, filed Mar. 28, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
In various fields relating to paints, inks, adhesives, textile processing and paper processing, polyurethane emulsions are used. To ensure excellent dispersion stability of the polyurethane emulsion, it is required that particles of the polyurethane emulsion have relatively small particle size and that dispersed particles of the emulsion are less likely to separate and settle over time and also are superior in storage stability.
As the process for producing a polyurethane emulsion having superior storage stability, there has been employed a process of premixing a urethane prepolymer with water containing an emulsifier using a simple propeller blade type mixer and finely dispersing the urethane prepolymer under high pressure conditions using a homogenizer.
When using a urethane prepolymer containing substantially no organic solvent in such a process, the resulting emulsion contains particles having relatively large particle size and poor storage stability over time. Therefore, a urethane prepolymer containing a certain amount of an organic solvent (in an amount of about 25 to 60 parts by weight based on 100 parts by weight of the solid content of the urethane prepolymer) must be used so as to obtain a polyurethane emulsion having superior storage stability.
Thus, it is necessary to perform the step of removing an organic solvent, which is optionally contained, so as to obtain an environmentally friendly polyurethane emulsion containing substantially no organic solvent.
The above process of the prior art had problems in that it requires numerous steps for fine dispersions and also requires the step of removing an organic solvent because the organic solvent is contained, resulting in poor production efficiency. Furthermore, the organic solvent contained in the urethane prepolymer remains in the polyurethane emulsion.
To solve these problems, there is proposed a process for continuously emulsifying by supplying a mixed solution of a urethane prepolymer and water in an emulsifying apparatus comprising a rotor section having multiple rotor teeth fixed to a rotational shaft provided in a cylindrical casing and a stator section having multiple stator teeth, which stand face to face with the rotor teeth, the stator section having an inlet, (see, for example, Japanese Patent Application, Second Publication No. Hei 7-68355 (pages 2 to 5)).
According to this process, an emulsion of a urethane prepolymer comprising fine particles can be obtained by continuously emulsifying a certain urethane prepolymer; however, there cannot be obtained a polyurethane emulsion which contains substantially no organic solvent and also has excellent storage stability over time. There is a problem in that the resulting emulsion has poor storage stability when using a urethane prepolymer which has an isocyanate group and has no hydrophilic group.
As the urethane prepolymer, a urethane prepolymer having a hydrophilic group such as ionic group, or a urethane prepolymer which has no hydrophilic group or has a very small content of hydrophilic group is used according to the purposes, and there is required a process capable of producing satisfactory polyurethane emulsion even when using any urethane prepolymer. Among the above urethane prepolymers, as the content of the hydrophilic group in the urethane prepolymer increases, the particle size of the polyurethane emulsion obtained by emulsification tends to become small.
In the above process for continuously emulsifying (see, for example, Japanese Patent Application, Second Publication No. Hei 7-68355 (pages 2 to 5)), when using a urethane prepolymer having no hydrophilic group, or a urethane prepolymer having a very small content of the hydrophilic group wherein the content of an ionic group such as anionic group or cationic group, as a hydrophilic group, is 0.01 equivalents or less based on 100 parts by weight of the urethane prepolymer, or the content of a nonionic hydrophilic group is 5 parts by weight or less based on 100 parts by weight of the urethane prepolymer, the particle size of the emulsion cannot become sufficiently small.
When using a urethane prepolymer wherein the content of the nonionic hydrophilic group is 10 parts by weight or more based on 100 parts by weight of the urethane prepolymer, the particle size can become small to some extent. However, the viscosity of the resulting polyurethane emulsion increases, and therefore the solid content of the polyurethane emulsion is limited to about 30% by weight or less and high concentration polyurethane emulsion cannot be obtained.
Thus, this process had such a problem that it is impossible to continuously produce a polyurethane emulsion which contains substantially no organic solvent and also has excellent storage stability over time.